The purpose of this clinical trial is to evaluate the ability of various biomarkers measured in serum and/or pleural fluids to estimate the risk of finding cancer in patients with undiagnosed pleural effusions. An algorithm for prediction of the risk of finding cancer in the patient using an optimal combination of markers will be developed, and patients will be categorized as having a low, moderate, or high risk of finding cancer that might be used to more effectively triage patients.